This invention relates to optical indexing discs, and more particularly, to a self-centering disc. Optical encoding discs which have rings of slits around their periphery may be mounted on a hub and the hub, in turn, mounted on a shaft. It is important that the axis of the disc be positioned precisely concentric with the axis of the shaft. Since the diameter of different shafts may vary, the discs have often been constructed with central holes slightly larger than the largest expected shaft, and the radial positions of the discs have been adjusted by hand to position them precisely on center with the shafts. Such positioning of the discs has been time consuming. A simple and low cost disc which precisely centers itself would be of considerable value.